The principal difference in dynamos is, perhaps, more clearly illustrated by the diagrams shown in Figs. 13, 14, 15, and 16. In Fig. 13 the arrangement of armature and field-magnet is the same as in the uni-direction machine, the field (F) being of magnetized steel, while the armature (A) is of soft iron wound with coils of fine wire, the ends of which are brought out at the commutators (C), through which the current is carried to the brushes (B and B B). If, however, the soft iron cores are used, a separate magnetizing electric current must be passed through the coils of wire wound about the field-pieces, so that they will become temporary magnets--the same as the cores of an electric bell movement, a telegraph-sounder, or the induction-coil core when a current is passed through the primary coil. The armature (A) is then driven at high speed by power, and the current is taken off for use through wires that lead from B and B B. In all of these figures the armatures rotate, in the space between the large pole-pieces of the field-magnets, in the same direction as the hands of a clock move. In these figure drawings the field-magnets, commutators, and brushes only are shown, the armature being indicated by the circle (A). Figure 13 represents a dynamo, the field-magnets of which are excited by a separate battery or generator. This is known as a “separately excited” machine, and is employed for various uses. The brushes (B and B B) are connected to the external circuit--that is, with the motor or other apparatus for which current is to be generated. The magnetic field in which the armature rotates will be constant if the exciting current is constant, like the magnetism in the magnet of the uni-direction current machine. The induced electro-motive force (which depends upon the rate at which the lines of force are cut) will be constant for the given speed at which the armature rotates. This action is the same as that described for the uni-direction current machine. Figure 14 is the diagram of a “series”-wound dynamo. The field and armature are soft gray iron, and are wound in series--that is, one end of the magnet-winding is made fast to the brush B, the other to the brush B B, and the apparatus to be operated by the current is let in between B B and the magnet, as shown by the indicated electric arc-light in the illustration. The field-magnet coils, the armature, and the external conductors are in series with each other, forming a simple circuit. When the armature is driven at high speed the field-magnets become self-exciting, with the result that current is generated. Its simple course is through B B to commutators on the hub, thence through one winding on the iron armature A, to B, through field F, and back to B B again, operating in its course any pieces of equipment designed for electric impulse, such as motors, or lamps, trolley-cars, trains, or electric machinery. [Illustration: FIG. 13] [Illustration: FIG. 14] [Illustration: FIG. 15]